CNS neuropeptide systems, peptide degradation and stress related diseases daptive responses initiate sequential steps of transmitter release in the CNS with corticotropin releasing hormone (CRH) being a key integrator (Dunn and Berridge, 1990; Koob and Heinrichs, 1999). Other neurotransmitters may modulate the course and outcome of CRH-induced behavioral, endocrine and immunological alterations. It has been demonstrated that endogenous neuropeptide Y (NPY) exert anti-CRH-like effects (Heilig et al., 1994; Thorsell et al., 1999; Britton et al., 2000). Recent clinical data implicate CRH in the etiology and pathophysiology of a variety of endocrine, psychiatric, neurodegenerative, and immunological disorders (Behan et al., 1995; Dieterich et al., 1997; Linthorst et al., 1997; Owens and Nemeroff, 1991; Wilder, 1993). Apart from selective receptor blockade of CRH receptors, an increase of endogenous anti-CRH-like acting NPY may therefore be beneficial. The relative increase of an endogenous NPY-like tone may be pharmacologically achieved by either increased degradation of CRH or by inhibition of the degradation of NPY. NPY is a substrate for the enzyme DPIV. A modulation of CNS DPIV-like activity provides, therefore, a new treatment regime of neurological and neuropsychological disorders.
DPIV and NPY
DPIV (CD26; EC 3.4.14.5) is an ectopeptidase with a triple functional role. DPIV is involved in truncation of Xaa-Pro dipeptides, circulating hormones and chemokines (Mentlein et al., 1999; Pauly et al., 1999), in T cell dependent immune responses (Kahne et al., 1999; Korom et al., 1997) and in metastasis (Cheng et al., 1998; 2000). DPIV selectively cleaves peptides after penultimate N-terminal proline and alanine residues. Endogenous substrates for this enzyme include the incretins, such as glucose-dependent insulinotropic polypeptides, like GIP and GLP-1. In the presence of DP IV, these hormones are enzymically degraded to inactive forms. NPY is one of the best, if not the best, substrates of DPIV-like enzymically activity (Mentlein, 1999). So far, the function of DPIV-like enzymatic activity within the CNS is not understood nor is the modulation of CNS DPIV-like activity the objective of any pharmacological treatment regime.
Neuropeptide Y, peptide YY and pancreatic polypeptide share an evolutionary conserved proline-rich N-terminal sequence, a structure generally known to be inert to the attack of common proteinases, but a potential target for specialized proline-specific aminopeptidases. Mentlein et al. examined purified human DPIV, that liberated N-terminal Tyr-Pro from both, neuropeptide Y and peptide YY, with very high specific activities and Km values in the micromolecular range, but almost no Ala-Pro from pancreatic polypeptide. Other proline-specific aminopeptidases exhibited low (aminopeptidase P) or totally no activity (dipeptidyl peptidase II). When human serum was incubated with neuropeptide Y or peptide YY at micro- and nanomolar concentrations, Tyr-Pro was detected as a metabolite of both species. Formation of Tyr-Pro in serum was blocked in the presence of Lys-pyrrolidine and diprotin A (Ile-Pro-Ile), specific competitive inhibitors of dipeptidyl peptidase IV. Incubation of neuropeptide Y or peptide YY with immunocytochemically defined, cultivated endothelial cells from human umbilical cord also yielded Tyr-Pro. Dipeptidyl peptidase IV could be immunostained on most endothelial cells by a specific antibody. They suggest, that dipeptidyl peptidase IV might be involved in the degradation of neuropeptide Y and peptide YY to N-terminal truncated neuropeptide Y (3-36) and peptide YY (3-36). Since specific binding to Y1, but not to Y2 subtype of neuropeptide Y/peptide YY receptors requires intact N- as well as C-termini of neuropeptide Y and peptide YY, removal of their amino-terminal dipeptides by dipeptidyl peptidase IV inactivates them for binding to one receptor subtype (Mentlein et al. 1993).
Discovery of NPY
Neuropeptide Y (NPY), a 36 amino acid peptide belonging to the pancreatic polypeptide family, was first isolated from porcine brain in 1982 (Tatemoto and Mutt, 1982). NPY is present in all sympathetic nerves innervating the cardiovascular system and is the most abundant peptide in the brain and the heart. Additionally, in rats, but not in humans, NPY is also found extraneuronally in platelets and endothelium (Zukovska-Grojec et al., 1993). Originally, NPY was known as a potent vasoconstrictor and a neuromodulator. Released by stress, exercise, and myocardial ischemia, NPY has been implicated in coronary heart disease, congestive heart failure, and hypertension (Zukovska-Grojec et al, 1998). More recently, because of the potent ability of NPY to stimulate food intake, it is suspected to play a role in obesity and diabetes (Kalra et al., 1999). Latest findings indicate, that NPY is also a mitogen for rat aortic vascular smooth muscle cells (Zukovska-Grojec et al., 1999).
NPY-related research has focussed on at least three main directions: (1) Co-transmission and sympathetic vasoconstriction, because of its co-expression with noradrenaline; (2) neurotransmission and function within the CNS, because of potent consummatory effects; and (3) evolution of NPY, since NPY is one of the most highly conserved bio-active peptides known (Colmers and Wahlestedt, 1993; Lundberg, 1996; Wahlestedt and Reis, 1993; Wettstein et al., 1996). NPY acts on at least six receptors (Y1-Y6), with varying peptide pharmacology and distinct distribution in the CNS (Gehlert, 1998) (Tab. 1).
Distribution of NPY, NPY Receptor Subtypes and mRNA
The distribution of NPY itself, NPY receptor protein and their mRNA within the CNS of human and rat brains has recently been reviewed (Dumont Y, Jacques D, St-Pierre, J.-A., Tong, Y., Parker, R., Herzog H. and Qurion, R., 2000; in Handbook of Chemical Neuroanatomy, Vol. 16: Peptide Receptors, Part I; Quirion, R., Björklund, A. and Hökfeld, T., editors). A brief survey is given in Tab. 1.
NPY-containing neurons are evident in the nasal mucosa of various species including man, often associated with glandular acini and blood vessels (Baraniuk et. Al., 1990; Grunditz et. al., 1994). Stimulation of the parasympathetic nerve supply to the nasal mucosa (vidian nerve) in dogs increases blood flow in the region and causes mainly atropine resistance. Intravenous administration of NPY reduces vasodilitation due to parasympathetic nerve stimulation, an effect that was not mimicked by the NPY Y1-selective agonist [Leu31, Pro34]NPY, but was mimicked by administration of the NPY Y2-receptor agonist N-acetyl[Leu28,Leu31JNPY(24-36) (Lacroix et al., 1994). This is consistent with a prejunctional NPY Y2-like receptor-mediated inhibition of transmitter release from parasympathetic nerve terminals.
NPY Receptor Function
Since the discovery of NPY in 1982, it became apparent that NPY is involved in the regulation of several behavioral and physiological functions (Colmers and Wahlestedt, 1993; Wettstein et al., 1996) (Tab. 1). In the brain, NPY has been implicated in anxiety and depression, feeding and obesity, memory retention, neuronal excitability, endocrine function, and metabolism (Gehlert, 1998). NPY is unarguably the most abundant neuropeptide discovered to date, with a wide distribution in the CNS and the peripheral nervous system (PNS). NPY forms a family of peptides together with peptide YY (PYY) (approximately 70% homology) and pancreatic polypeptide (PP) (approximately 50% homology); both NPY and PYY are extremely bio-active, whereas PP is generally much less active (Gehlert, 1998; Wahlestedt and Reis, 1993) (Tab. 2).
Receptors for neuropeptide Y are also located on sensory nerve terminals and their activation can modulate local neurogenic responses (Grundemar et al., 1990; 1993). Two receptor subtypes have been called neuropeptide Y Y1 (postjunctional) and neuropeptide Y Y2 (prejunctional) on the basis of the different responses to a truncated analog of the related peptide YY-(13-36), when compared with neuropeptide Y in in vitro assay systems (Wahlestedt et al., 1986). Activation of neuronal prejunctional NPY receptors generally inhibits nerve activity, reducing the release of neurotransmitters in response to nerve impulses and in response to local factors acting to release neurotransmitters (Wahlestedt et al., 1986). The prejunctional or neuropeptide Y Y2 receptor classification was based on actions of peptide YY (13-36) but in many systems this molecule, as well as neuropeptide Y-(13-36), does exhibit pressor activity (Rioux et al., 1986; Lundberg, et al., 1988; Potter et al., 1989). This has been interpreted by some to indicate that in some vascular beds there are two types of neuropeptide Y receptors (both neuropeptide Y Yj and neuropeptide Y2) on postjunctional membranes (Schwartz et al., 1989). However the lack of selectivity of these molecules may be due to retention of partial agonistic activity on Yj receptors, which permits them to evoke a reduced functional response. Previously, a 13-36 analog of neuropeptide Y, (Leu 17, Glu″, Ala 21, Ala 22, Glu 23, LeU28, LeU31) neuropeptide Y-(13-36) (ANA neuropeptide Y-(13-36)) which displayed prejunctional activity equivalent to the whole neuropeptide Y molecule in studies in vivo was described (Potter et al., 1989).
Apart from these historically well-defined neuropeptide Y receptors the existence of a number of other subtypes (Y3, Y4, Y5 and Y6) has been suggested on a pharmacological basis (Michel et al., 1998) and details of the cloning of receptors corresponding to Y1, Y2, Y4 and Y5 have been published (Herzog et al., 1992; Gerald et al., 1995; Bard et al., 1995; Gerald et al., 1996) (Tab. 1). The distribution and physiological significance of these various receptor subtypes has yet to be defined. Although some controversy has existed about the selectivity of truncated forms of neuropeptide Y for one or other receptor subtype (Potter et al., 1989), the emerging picture supports the initial classification into pre- and postjunctional receptor subtypes. Cell lines have been developed which express specifically one neuropeptide Y receptor subtype and the development of receptor-selective analogs of neuropeptide Y has focussed mainly on binding characteristics in these cell lines (Sheikh et al., 1989; Aakerlund et al., 1990; Fuhlendorff et al., 1990). More recently, a cDNA encoding the neuropeptide Y Y1 receptor has been cloned and cell lines expressing the cloned receptor have been analyzed for both specific binding of neuropeptide Y analogs (Herzog et al., 1992) and functional responses elicited by specific analogs. From such binding studies, combined with subsequent studies in vivo, two analogs have been classified as acting specifically on the postjunctional neuropeptide Y Y1 receptor. These neuropeptide Y Y receptor selective analogs, (Pro 34) neuropeptide Y and (Leu″, Pro 34) neuropeptide Y, mimic the action of neuropeptide Y in raising blood pressure, and also share similar binding to cell lines expressing only neuropeptide Y Y receptors e.g. the human neuroblastoma cell line SK-N-MC and fibroblast lines expressing the cloned neuropeptide Y Y, receptor (Herzog et al., 1992). Neither exhibits the neuropeptide Y Y2 receptor action an inhibition of cardiac vagal action in vivo, a manifestation of inhibition of acetylcholine release (Potter et al., 1991; Potter and McCloskey, 1992).
TABLE 1DISTRIBUTION AND FUNCTION OF NPY RECEPTOR SUBTYPES WITHIN THE CNSReceptor-CNSSelective AntagonistsubtypeExpressionFunctionSelective Agonistor SelectivityY1Cortex, etc.Anxiolysis, LHRHIntact N - Terminus:BIBP3226; BIBO 3304Release[Leu31, Pro34]NPYY2Hippocampus,AntiamnesticC-terminale End:T4[NPY(33-36)]4;HypothalamusPYY3-36; PYY13-36BIIE0246Y3Ncl. TractusBradycardia,NPY>>PYY,PYY - InsensitivitySolitarius (NTS)Hypotension[Leu31, Pro34]NPYY4Dorsal vagalEmeticPP>>NPY, PYYPP - PreferringComplex (DVC)Y5 (a)HypothalamusFeedingNPY, PYY,[Leu31, Pro34]NPY -[Leu31, Pro34]NPYsensitive, BIBP3226 -non-reversibleY5 (b) or Y6Hypothalamus?; species specific??Tab. 1: NPY Receptor subtypes within the CNS; ? = unknown or not investigated
The development of the high affinity, non-peptide NPY antagonists, BIBP3226 and BIB03304, has facilitated the functional characterization of NPY receptors, as this compound shows selectivity for Y1R, being devoid of activity on at least Y2R, Y3R and Y4R (Doods et al., 1996). Recently, a two Y2 receptor antagonist has been described. One is a TASP-molecule (Grouzmann et al., 1997), the other a non-peptide antagonist (Wieland et al., 1999) and other non-peptide receptor specific compounds became available (Daniels et al., 1995). Thus, specific receptor blockade within the brain would allow the functional characterization of behavioral and physiological effects mediated by central NPY receptors. In addition, mice lacking the Y1R were generated and are available (Pedrazzini et al., 1998). Neurons showing NPY-like immunoreactivity and NPY receptor expression are abundant in the CNS (Tab. 1), and perhaps are most notably found in hypothalamic and so-called limbic structures, but are also co-localized with brain stem monoaminergic neurons and cortical GABA-ergic neurons (Chronwall, 1985; Dumont et al., 1996). The latter may be of particular importance, because the GABA-benzodiazepine receptor complex is an important negative modulator of CRH secretion and of responsiveness to excitatory stimuli in rats and humans (Gear et al., 1997; Smith et al., 1992; Judd et al., 1995).
TABLE 2RECEPTOR SUBTYPES UND PEPTIDE SELECTIVITYReceptorsubtypePeptide PotencyY1-likeY1NPY = PYY = Pro34-NPY > PP > NPY13-36Y4PP >> NPY = PYY = LP-NPY > NPY13-36Y6NPY = PYY = Pro34-NPY > NPY13-36 > PPY2-likeY2NPY = PYY = NPY13-36 > Pro34-NPY > PPY5-likeY5NPY = PYY = Pro34-NPY > NPY13-36 > PPNot clonedPP receptorPP >> PYY = NPYY3NPY = Pro34-NPY = NPY13-36 >> PYYPYY-PYY > NPY >> NPY13-36 >> Pro34-NPYpreferringTab. 2: Receptor subtypes and peptide selectivity according to Gehlert, 1998.
As has to be demonstrated, most of the central NPY effects are opposite to those observed after CRH application, stress or those which are found in anxiety related disorders. NPY almost completely resemble the effects produced by benzodiazepine application.
NPY and Autonomic Regulation
With respect to autonomic regulation, the results of Egawa et al. and others on i.c.v. CRH- and i.c.v. NPY- (Egawa et al., 1990; 1991; van Djik et al., 1994) mediated effects on sympathetic firing rate to brown adipose tissue (IBAT) demonstrate that CRH increases while NPY reduces the sympathetic outflow. These effects support the anti-stress-like functional role of CNS NPY systems.
NPY and Immune Functions
The immune system is also affected by NPY. Here, similarly to CRH-mediated effects, the effects of NPY could be subdivided into direct and indirect (centrally) mediated effects (von Hörsten et al., 1998a). I.c.v. applied NPY, and derived peptides, affect innate immune function, IL-6 levels, and leukocyte subsets, and these effects display dose, time, receptor, and compartment specificity (von Hörsten et al., 1998 a, b, c). Since NPY immunoreactivity increases in the brain after peripheral induction of acute monoarthritis (Bileviciute et al., 1995), increased brain NPY levels may reflect partly an adaptive response to changes induced by inflammation. Importantly, the long lasting immunostimulatory action of i.c.v. NPY parallels the effects of Methionine-Enkephalin (MET-ENK) (von Hörsten et al., 1998c). Thus, while central CRH appears to be a key mediator of stress effects on the innate immune system (Irwin, 1994), NPY may interact with CRH or even antagonize its effects. The benzodiazepine-like action of NPY in conjunction with data demonstrating that benzodiazepines abrogate CRH-induced suppression of NK cell function (Irwin et al., 1993), further support the hypothesis, suggesting an anti-CRH-like, “stress-protective” action of NPY receptors activation.
NPY and Central Cardiovascular Regulation
The highest levels of NPY Y1, Y2, Y4 and Y5 receptors are found in the nucleus tractus solitarius (NTS), the area postrema and the dorsal vagal complex in the rat brain. These receptors are likely to be involved in the CNS-mediated effects of NPY on various cardiovascular and respiratory parameters (Dumont et al, 1992; McAuley et al., 1993). For example, direct injections of NPY into the NTS produce vasodepressor effects and suppressed baroreceptor reflexes (Grundemar et al, 1992; Shih et al., 1992). These effects may be mediated via the Y2 or the Y3 receptor based on the relative potency of NPY13-36 (Narvaez et al, 1993) but no potency of PYY (Grundemar et al., 1991a, b). Thus, within the CNS, brainstem NPY systems may exert anti-hypertensive effects. These central effects of NPY appear to be opposite to the periphery. In the peripheral cardiovascular system, NPY raises blood pressure by an action on postjunctional Y receptors and inhibits neurotransmitter release—both acetylcholine and noradrenaline—by acting on prejunctional neuropeptide Y receptors. When administered intravenously, NPY produces a potent and long-lasting vasoconstriction that is not blocked by alpha or beta adrenergic antagonists (Wahlestedt et al., 1986).
NPY and Thermoregulation
Potent hypothermic effects of NPY have been described (Esteban et al., 1989; Jolicoeur et al., 1991; Currie and Coscina, 1995). Interestingly, Y1 receptor antisense-treated rats demonstrated increases in body temperature (Lopenz-Valpuesta et al, 1996), suggesting that the Y1 receptor subtype could be responsible for the hypothermia induced by NPY. No further studies or pharmacological approaches explored the possibility that NPY Y1 receptor activation might be useful in the treatment of fever.
NPY, Circadian Rhythms and Sleep
The suprachiasmatic nucleus in conjunction with the geniculo hypothalamic tract is of critical importance in diurnal rhythms (Albers and Ferris, 1984; Meijer and Reitveld, 1989). The effect of NPY on circadian rhythms is believed to be mediated in the suprachiasmatic nucleus (Biello et a., 1994; Human and Albers, 1994) and the Y2 receptor subtype has been implicated in the effect (Golombeck et al., 1996; Human et al., 1996) by modulating glutamatergic neuronal activity (Biello et al., 1997). However, considering the effect of NPY on GABAergic neurons in the suprachiasmatic nucleus (Chen and van den Poi, 1996; Biggs and Prosser, 1999), it appears that other NPY receptor subtypes could also play a role in modulation of circadian rhythms.
Disturbance of sleep is a common health problem and often associated with depression. In rats i.c.v. NPY treatment has been demonstrated to overcome CRH-induced and stress-induced shortening of sleep (Yamada et al., 1996). With regard to sleep regulation in humans, Ehlers et al., (1997) found that “dysregulation” of sleep and arousal states in depression and anxiety may be consistent with an upset of the balance between hypothalamic neuropeptide systems for NPY and CRH. Antonijevic et al. (2000) reported that NPY promotes sleep and inhibits the hypothalamo-pituitary-adrenocortical (HP) axis in humans, pointing to a possible role of NPY agonists for the development of novel treatment strategies for affective disorders. Since in major depression increased HPA-activity, sleeping disorder, anxiety and loss of appetite are main characteristics these findings further support a role of NPY in anxiety-disorders. Thus, a pharmacologically induced increased of NPY levels might exert sleep-promoting effects. Yet, pharmacological approaches to increase NPY levels are needed.
NPY and Nociception
Interestingly, NPY has also been reported to modulate nociception. There is evidence, that centrally (i.c.v.) applied NPY induces hyperalgesic effects on hot plate latency in mice (Mellado et al., 1996) and rats (von Hörsten et al., 1998c). These results also parallel the finding that benzodiazepines antagonize opioid and opiate analgesia via enhanced action of GABA at the GABA-A receptors (Gear et al., 1997). At a spinal level, in the dorsal root ganglia, NPY appears to exert analgesic-like effects and an increase of NPY Y2 receptor mRNA as well as NPY-like immunoreactivity has been reported after sciatic nerve lesions (Zhang et al., 1993). However, the physiological role of NPY in nociception remains to be established.
NPY and Feeding
On a behavioral level, most of the research has focussed on the potent orexigenic effects of NPY (Clark et al, 1984; Marsh et al., 1998; Kalra et al., 1999; O'Shea, et al., 1997; Stanley and Leibowitz, 1985). The orexigenic effect of NPY parallels the known orexigenic “side” effect of benzodiazepine treatment, and is opposite to the anorexigenic effect of CRH. CRH and NPY antagonize their feeding effects (Heinrichs et al., 1993; Menzaghi et al., 1993). I.c.v. CRH stopped weight gain in genetically obese (fa/fa) NPY overexpressing rats (Bchini-Hooft et al., 1993), and it was shown that the hypothalamic NPY feeding system is largely dependent on circulating corticosterone (Stanley et al., 1989). Chronic i.c.v. infusion of NPY has been demonstrated to decrease hypothalamic content of CRH (Sainsbury et al., 1997). Possibly, the induction of hunger by increased hypothalamic NPY content affects other motivational systems. Structure-affinity and structure-activity relationship studies of peptide analogs, combined with studies based on site-directed mutagenesis and anti-receptor antibodies, have given insight into the individual characterization of each receptor subtype relative to its interaction with the ligand, as well as to its biological function. A number of selective antagonists at the Y1-receptor are available whose structures resemble that of the C-terminus of NPY. With respect to the behavioral regulation of feeding behavior, some of these compounds, like BIBP3226, BIB03304 and GW 1229, have recently been used for in vivo investigations of the NPY-induced increase of food intake (Cabrele and Beck-Sickinger, 2000) and is was found that probably Y1 and Y5 receptors are involved in the mediation of these effects (Wieland et al., 1998).
NPY, Anxiety and Depression
Anxiolytic-like effects of NPY have been demonstrated using the elevated plus maze test (Montgomery), the punished drinking test (Vogel), and the punished responding test (Geller-Seifter), with potency and efficacy matching those of benzodiazepines (Griebel, 1999; Heilig et al., 1989; Wettstein et al., 1995). NPY acts anxiolytic-like on the response to novelty (Heilig and Murison, 1987; von Hörsten et al., 1998b), and produces anxiolytic-like effects on the elevated plus maze and other anxiety related tests (Wahlstedt and Reis, 1993; Wahlestedt et al., 1993). Interestingly, Y1 receptor antisense-treated rats showed marked anxiety-related behaviors, without alterations of locomotor activity and food intake (Wahlestedt et al., 1993). Additionally, in the Flintier rat strain, a genetic model of depression, Y1 receptor mRNA expression was decreased in different cortical regions and the dentate gyms of the hippocampus, while Y2 receptor mRNA expression did not differ from controls (Caberlotto et al., 1998). Olfactory bulbectomy in the rat has been developed as a model of depression (Leonard and Tuite, 1981). In this model, most of the changes resemble those found in depressed patients (Song et al., 1996). A 7-day i.c.v. administration of NPY in olfactory bulbectomized rats attenuated behavioral and neurotransmitters deficits in this model (Song et al., 1996). All these data argue for a role of NPY in anxiety-related disorders. NPY Y1, Y2, and possibly Y5 receptors, seem to be involved in the regulation of anxiety levels in rodents, with Y1-mediated effects being best characterized (Heilig et al., 1993; Kask et al., 1998b). Again, in comparison with benzodiazepines, anxiolysis is one of the most important properties of these compounds, especially of those affecting central CRH systems (e.g. Alprazolam) (Arvat et al., 1998; Korbonits et al., 1995; Kravitz et al., 1993; Torpy et al., 1993). It can be concluded, therefore, that endogenous NPY counteracts stress and anxiety (Heilig et al., 1994). Furthermore, these data suggest that the Y1 receptor subtype could be implicated in anxiety- and depression-related behaviors. Additionally, Kask et al. (1996) reported that i.c.v. injection of the Y1 antagonist, BIBP3226, produced anxiogenic-like effects in the elevated plus-maze test, without any locomotor deficit. This effect can be reproduced by the administration of BIBP3226 in the dorsal periaqueductal gray matter but not in the locus coeruleus of the paraventricular nucleus of the hypothalamus (Kask et al., 1998c). Moreover, BIBP3226 and GR231118 administered into the dorsal periaqueductal gray matter decreased the time spent in active social interaction in rats (Kask et al., 1998d). These data suggest that endogenous NPY, under stressful and non-stressful conditions, relieve anxiety via the Y1 receptor.
The brain regions which are important for the anti-stress action of NPY include but may not be limited to the amygdala (Sajdyk et al., 1999, Thorsell et al., 1999), locus coeruleus (Kask et al., 1998c) and dorsal periaqueductal gray (Kask et al., 1998a,b). Amygdala NPY is not released under low stress conditions since blockade of NPY Y1R with BIBP3226 or BIB03304 did not increase anxiety as measured in the elevated plus-maze and social interaction tests (Kask et al., 1998b; Sajdyk, 1999). Constant NPY-ergic tone, however, seems to exist in the dorsal periaqueductal gray matter, where the NPY Y1R antagonist had anxiogenic like effects in both experimental anxiety models (Kask et al., 1998a,b). Thus, in certain brain regions, there may be a tonic regulation of anxiety via NPY systems.
Interestingly, the levels of NPY in the CSF of patients with major depression were reduced as compared to non-depressed patients (Widerlov et al., 1986, 1988). Similarly, cortical tissues obtained form suicide victims with a medical history of depression revealed lower levels of NPY as compared to suicide victims with no reported depressive episodes (Widdowson et al., 1992). Higher levels of NPY in the CSF were found in depressed patients showing low symptoms of both psychological and somatic anxieties, while anxious patients had lower levels (Heilig and Widerlov, 1995). Most recently, lower plasma levels of NPY were reported in suicidal patients compared to healthy controls (Westrin et al., 1999). Taken together, these studies demonstrate that NPY is likely involved in anxiety-related behaviors in humans. However, at present, no pharmacological approaches are available to gain advantage of these beneficial effects of elevated NPY levels in anxiety-related disorders.
NPY, Seizures and Epilepsy
Having the similarities between NPY mediated effects and benzodiapines in mind, another important field for the treatment with benzodiazepines is their anti-convulsive property. Surprisingly, NPY deficient mice, despite otherwise largely normal phenotypes, exhibit spontaneous seizures (Erickson et al., 1996a,b), while exogenously administered NPY reduces the incidence and severity of kainic acid-induced seizures (Woldbye et al., 1997). Elevated NPY levels have been observed following limbic seizures, suggesting that it may have a protective effect against further seizure activity (Vezzani et al., 1996). Thus, another evolving role of NPY is found in neuronal excitability, and again, the parallelism with exogenous benzodiazepines is striking and the opposite effects to CRH-induced seizures (Ehlers et al., 1983) are apparent (Erickson et al., 1996; Vezzani et al., 1999).
In humans, temporal lob epilepsy is a neurological disorder in which the hippocampal formation is severely affected. In approximately two thirds of the cases, the hippocampus is often the only structure that shows pathological modifications (Amaral and Insausti, 1990). Considering that NPY-containing neurons degenerate in the hippocampus of patients with temporal lobe epilepsy (de Lanerolle et al., 1989) and that NPY regulates neuronal excitability in the rat hippocampus, the role of NPY and its receptors in humans certainly is critical. Thus, NPY and NPY receptor provide an important pharmacological target for the development of new anti-epileptic-like acting drugs. However, no compounds fulfilling pharmacological criteria for CNS targeting, peptide or receptor specificity and bioavailability are available at present.
NPY, Learning, Aging, Neurodegeneration with Cognitive Dysfunction
NPY and PYY enhance memory retention (Flood et al., 1989). The hippocampal Y2 receptor has been implicated in facilitating learning and memory processes with increases in memory retention induced by NPY (Flood et at., 1987). Passive immunization with NPY antibodies injected into the hippocampus induced amnesia (Flood et al., 1989). The hippocampal formation is associated with learning memory processes and is an area severely affected in Alzheimer's disease (Terry and Davies, 1980). Several studies have reported significant decreases in NPY-like immunoreactivity in cortical, amygdaloid and hippocampal areas in Alzheimer's disease brains (Chan-Palay et al., 1986b). Moreover, NPY binding sites are reduced in cortex and hippocampus of patients suffering from Alzheimer's disease (Martel et al., 1990). These data suggest that the degenerative process occurring in Alzheimer's disease may involve changes in NPY-related enervation. Interestingly, a major loss in NPY-like immunoreactive neurons has been reported in aged rats especially in cortical areas (Cha et al., 1996, 1997; Huh et al, 1997; 1998) and hypothalamic release of NPY is decreased in older rats (Hastings et al., 1998). The direct impact of NPY losses on cognitive behaviors in Alzheimer's disease remains to be established. Similar, in other neurodegenerative disorders such as Huntington's disease selective changes of NPY changes have been reported (Ferrante et al., 1987; Beal et al., 1986). At present, no treatment approaches have focussed on increasing NPY concentration in neurodegenerative disorders and/or other diseases states associated with cognitive dysfunction.
NPY, Opioid Withdrawal and Alcoholism
The expression of opioid withdrawal is thought to involve various brain regions (Koob et al., 1992). Early studies have suggested that exogenous application of NPY and related agonists could antagonize withdrawal by correcting deficits in NPY-like immunoreactivity at the levels of the hypothalamus (Pages et al., 1991). Recently, i.c.v. injections of NPY and related peptides have been shown to attenuated motor scores alterations induced by naloxone-precipitated withdrawal from morphine in rats (Woldbey et al., 1998). It remains to be established if similar data could be obtained in humans. Very recently, experimental studies have suggested that NPY, together with its receptors, may have direct implication in addiction to alcohol. NPY is involved in the Modulation of ethanol consumption and resistance. NPY-knockout mice were shown to have high ethanol consumption and low sensitivity to ethanol (Thiele et al., 1998). In contrast, mice overexpressing NPY drank much less alcohol than wild type and were also more sensitive to ethanol (Thiele et al., 1998). At present, no pharmacological approaches are available to gain advantage of these likely beneficial effects of elevated NPY levels in withdrawal and alcoholism.
NPY and Schizophrenia
Emerging evidence suggests that NPY might be involved in the pathophysiology of neuropsychiatric disorders (Wettstein et al., 1995). Region-specific decrease in NPY content has been described in patients with schizophrenia (Frederikson et al., 1991; Widerlov et al., 1988). Overactivity of mesolimbic dopaminergic pathways is believed to be of importance in drug reinforcement and schizophrenia (Beninger, 1983). There is evidence that some effects of NPY might be mediated via activation of dopamine (DA) receptors. First, a number of behavioral effects of NPY can be blocked by DA receptor antagonists (Moore et al., 1994; Josselyn and Beninger, 1993). Moreover, NPY has been shown to stimulate NMDA-stimulated DA release from rat striatum (Ault and Werling, 1997) and nucleus accumbens (Ault et al., 1998) providing direct evidence that NPY potentiates dopaminergic neurotransmission in these brain regions. Kask and Harro (2000) found that NPY Y1 receptor antagonism inhibits amphetamine-induced hyperactivity in rats and concluded that the ability of NPY Y1 receptor selective antagonists to modulate behavioral response to amphetamine an apomorhine suggests that NPY Y1 receptors may be involved in mediation of psychosis and reinforcement.
Neurological and Psychophysiological Effects of CNS NPY Systems: Pleiotropy
Thus, numerous studies have addressed the physiological functions of NPY and its congeners in the CNS (for reviews see: Kalra and Crowley, 1992; Dumont et al., 1992; Stanley, 1993; Wahlestedt and Reis, 1993; Grundemar et al, 1993; Gehlert, 1994, 1998; Colmers and Bleakman, 1994; Wettstein et al, 1995; Heilig and Widerlowi, 1995; Munglani et al., 1996; Inui, 1999; Bischoff and Michel, 1999; Vezzani et al., 1999) and demonstrated a broad range of effects. This pleiotropy, together with its high degree of identity among mammalian species, suggests that NPY systems within the CNS are highly important pharmacological targets in various pathophysiological states. The peptide is involved in several neurological and psychophysiological processes, of which the anxiolytic-like, feeding, anti-convulsive and anti-addictive effects appear to be most prominent. These actions involve a site specific and a receptor specific action of NPY within the CNS. No pharmacological approaches exist, at present, to gain advantage of these various physiological functions.
Current Treatments of Anxiety Related Disorders Using Benzodiazepines
There are a number of new anxiolytic drugs that are fast acting and free from the unwanted side effects associated with the traditional benzodiazepines on the way toward the clinical development. Partial agonists at the benzodiazepine receptor, such as alpidem, abecarnil and bretazenil, have highly promising preclinical profiles and some useful preliminary results in clinical testing of anxiety disorder subjects. Neurosteroids are another interesting set of pharmacologic agents that target the benzodiazepine receptor, have a preclinical anxiolytic profile and now need to be tested in clinical populations.
Neuropeptide receptor agonists and antagonists with anxiolytic properties may represent one of the most striking new classes of the potential anxiolytic drugs. As described above in detail, preclinical studies as well as clinical studies suggest that agonists of the neuropeptide Y receptors are provocative targets for anxiolytic agents (Kunovac & Stahl, 1995).
Current Problems in the Treatment of Anxiety Related Disorders Using Benzodiazepines or NPY
The current methods for treatment of anxiety are accompanied by several problems:
The benzodiazepines that are commonly used as anxiolytic agents are unnatural compounds with a low or no selectivity. Beside their anxiolytic activity, the benzodiazepines show sedative and anti-epileptic effects and are suspected to influence muscle relaxation. Unfortunately, they are associated with a number of unwanted side effects, namely tiredness, sleepiness, lack of concentration, reduction of attentiveness and reactivity. Chronic application of benzodiazepines causes neurological disorders, like ataxia, dizziness, reflex loss, muscle and language disorders. A long-term treatment with benzodiazepines is predicted to entail dependency and addiction.
The direct i.c.v. administration of neuropeptide Y for the long-term treatment of anxiety in patients is not feasible.